The codex cover has been leaked, and the images are as of now floating across the internet everywhere. The cover was indeed given to us by a sneak peak during the How to Paint Wracks video earlier today from Games Workshop, and now the entire cover has leaked with Games Workshop's latest video: Warhammer 40,000: NEW Dark Eldar
The image above is a released image from Games Workshop from their video on the new Dark Eldar that can be seen following the link below.
Warhammer 40,000: NEW Dark Eldar
Dark Eldar - How to paint a Wrack
Other places you can see more images.